The subject invention provides improved means to measure mass collected on a sample collection surface in real time. A longitudinal resonant structure is used as the “scale” to determine the mass in real time. The invention employs a membrane, or collection substrate, on the end of a resonant structure resonating in the longitudinal direction. A structure resonating in the longitudinal mode offers a considerably higher resonant frequency than the same structure in the bending mode. The higher the resonant frequency, the more resolution in the frequency of oscillation is available to determine the mass of the collection surface or membrane. The present invention implements this attribute in connection with such a resonant structure utilizing various novel driving and sensing methods disclosed herein. Classical real time mass measuring systems utilize resonant structures in the bending mode as disclosed in patents by Korpi in U.S. Pat. Nos. 6,444,927 and 6,784,381 as well as U.S. patent application Ser. No. 10/904,074, U.S. patent application Ser. No. 10/188,823 and US Pat. App. No. 20060086174. An important improvement of this invention is to resonate the structure in the longitudinal direction as opposed to the bending mode described in the above mentioned patents and applications.
Aspects of the invention include improvements over known devices in terms of the materials, mounting, driving, and sensing methods of the resonant structure, tube, or resonator.
The subject resonant structures are typically driven in longitudinal resonance by one of at least two different excitation methods disclosed. One such means is a voice coil with magnet; another is a piezoelectric excitation method utilizing a piezoelectric “motor” to drive the system into resonance. An improvement in the art is that neither of these methods relies on the magneto restrictive qualities of the resonant structure. The Q, or the inverse measure of energy lost per cycle of resonance, of the materials disclosed herein are among the highest of all currently available materials.
The relative sensitivity to mass detection of inertial mass sensors is a function of the fundamental resonant frequency of the resonator. The greater the resonant frequency, the greater the sensitivity. The ability to accurately measure the mass collected, or lost, where the change in mass as delta-m, at the collection means is indicated by a shift in the resonant frequency.
Mechanical resonators in oscillation will have a small jitter in instantaneous resonant frequency, defined as delta-f, where this jitter is generally a function of the mechanical resonant Q of the system where the frequency jitter, df, divided by the oscillation frequency, f, or delta-f over f (df/f) and is roughly equivalent to inverse of the value Q for the system. The higher the Q the lower the frequency jitter will be. One material with one of the lowest jitter values, and one with one of the highest Q, is an amorphous structure such as fused quartz, which is exactly why these materials are used to make the crystals used in computers. A 2.66 GHz computer, for example, uses a 2.66 GHz crystal, which refers to the crystal clock used to clock the microprocessor that allows the computer to run. Another material with a high Q as well as high strength is an amorphous structure of metal that is cooled at a rapid rate to produce a glass like structure with metal properties as well as glass like properties, in short a metal like structure with an amorphous atomic structure.
As taught herein, resonant frequency is measured by measuring the frequency at a chosen sampling rate and averaging that frequency over N measurements which permits a reduction in the RMS jitter of the frequency which results in a more accurate determination of the resonant frequency, and therefore the measured mass of the collection surface or media which is a subject of this invention.
Advantageously, the highest frequency mode of oscillation of a resonant structure is in the longitudinal mode along the long axis, which is simultaneously the stiffest axis. Unique drive structures are described to enable driving a resonant structure in this mode.
The stability and resolution of the resonant frequency of oscillation, which in practice is always some net aggregate of frequencies (because several eigenmodes of oscillation are excited by the driving impulse or mechanical impulse) is dependent upon the mechanical resonant Q of the system.
Generally, the higher the Q of the system and the more pure the drive and resulting excitation of the resonator in the target mode of resonant oscillation the more accurately the resonant frequency of oscillation, and therefore the resulting mass, can be determined. For systems with a high mechanical Q and low frequency drift or jitter, also referred to as delta-frequency, the more accurately and repeatability the actual collected mass can be determined. Key to the subject teachings are techniques to maintain very high mechanical resonant Q of the entire system and therefore the resolution of mass change or detection of the mass at the collection means or filter.
Advantages of nodal clamping and the significant improvements of the mechanical resonant Q that result are taught herein. This teaching includes stiffening the filter collection means to minimize losses from conformal or ductile dampening materials, and introduces general fabrication practices to minimize internal and external frictional dampening that tend to lower the mechanical resonant Q as described in US Pat. App. No. 20100258357.